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guilt would be zero also, rendering a trial unnecessary. The courts of
several states have remained unmoved by this argument.
8
In State v. Spann,
9
showing the defendant had fathered the victim's
child was key to establishing a charge of sexual assault. The State's expert
testified that only 1% of the presumed relevant population of possible
fathers had the type of blood and tissue that the father had and, further,
that the defendant was included within that 1%. In other words, 99% of
the male population at large was excluded. Next, she used Bayes' Theorem
to show that the defendant had a posterior probability of fathering the
victim's child of 96.5%.
“The expert testifying that the probability of defendant's paternity
was 96.5% knew absolutely nothing about the facts of the case
other than those revealed by blood and tissues tests of defendant,
the victim, and the child . . .”
10
“In calculating a final probability of paternity percentage, the
expert relied in part on this 99% probability of exclusion. She also
relied on an assumption of a 50% prior probability that defen-
dant was the father. This assumption [was] not based on her
knowledge of any evidence whatsoever in this case ...[she stated]
“everything is equal ...he may or may not be the father of the
child.”
11
“Was the expert's opinion valid even if the jury disagreed with the
assumption of .5 [50%]? If the jury concluded that the prior prob-
ability is .4 or .6, for example, the testimony gave them no idea of
the consequences, no knowledge of what the impact (of such a
change in the prior probability) would be on the formula that led
to the ultimate opinion of the probability of paternity.”
12
“. . . [T]he expert's testimony should be required to include an
explanation to the jury of what the probability of paternity would
be for a varying range of such prior probabilities, running for
example, from .1 to .9.”
13
8
See, for example, Davis v. State, 476 N.E.2d 127 (Ind. App. 1985) and Griffith v. State of
Texas, 976 S.W.2d 241 (1998).
9
130 N.J. 484 (1993)
10
Id. 489.
11
Id. 492.
12
Id. 498.
13
Id. 499.
